1. Field of the Invention
The present invention generally relates to a fuse structure and more specifically to an inductive fuse.
2. Description of the Related Art
Conventional systems utilize fuses in semiconductor chips to provide redundancy, electrical chip identification and customization of function. For designs having three (or more) layers of wiring, the fuses are typically formed from a segment of one of the wiring layers, e.g. the xe2x80x9clast metalxe2x80x9d or xe2x80x9clast metal minus onexe2x80x9d wiring layer. Fusing (i.e., deletion of a segment of metal fuse line) is accomplished by exposing the segment to a short, high intensity pulse of xe2x80x9clightxe2x80x9d from an infra-red laser. The metal line absorbs energy, superheats, melts and expands, and ruptures any overlaying passivation. The molten metal then boils, vaporizes or explodes out of its oxide surroundings, disrupting line continuity and causing high electrical resistance. A xe2x80x9csensingxe2x80x9d circuit is used to detect fuse segment resistance. Sense circuits can be designed to xe2x80x9cunderstandxe2x80x9d that fusing has occurred when line resistance increases or line resistance decreases.
To improve signal propagation and overall circuit performance, high conductance materials such as copper are used for interconnect wiring. Specifically, wire resistance is reduced by using copper and this results in a reduction of the RC character of the circuit. Further reduction of RC can be achieved by using low dielectric constant (K) materials as the dielectric between adjacent wiring lines. Examples of low K dielectric materials include porous glass and polyimide nanofoams.
While the above combination of materials yields high performance integrated circuit devices, the low K dielectric materials are mechanically and thermally fragile and can be damaged or collapse under standard laser fuse blow conditions, and the high conductance metals such as copper or silver readily corrode if left exposed to the atmosphere. Dielectric damage and corrosion of the exposed metal can result in degradation of integrated circuit device yield and reliability.
Therefore, there is a need for a new type of fuse structure which can be blown, e.g. opened, without suffering the reliability problems described above.
It is, therefore, an object of the present invention to provide a structure and method for blowing a fuse that includes removing an insulator above a fuse link and etching the fuse link. The etching includes wet etching or reactive ion etching. The removing includes ablating the insulator using light energy. The light energy includes a laser light or ultra violet light.
Another inventive method is an inductive circuit in which an inductance is alterable. The inductive circuit includes a primary turn, a secondary turn and a shorted turn. Removing a portion of the shorted turn changes the inductance. The programming device is a fuse and the shorted turn is a fuse link. The secondary turn is positioned between the primary turn and the shorted turn. A current in the shorted turn prevents coupling between the primary turn and the secondary turn. The primary turn and the second turn include aligned openings and the shorted turn includes a portion extending over the aligned openings.
Another aspect of the invention is a fuse that includes a primary turn, a secondary turn and a shorted turn, wherein removing a portion of the shorted turn changes an inductance of the fuse. The primary turn includes a first wire positioned in a first plane, the first wire having two ends and an inner and an outer periphery, the wire having a loop shape such that the two ends are in close proximity to one another. The secondary turn includes a second wire position in a second plane, the second wire having two ends and an inner and an outer periphery, the second wire having a loop shape such that the ends are in close proximity to one another, the inner periphery of the second wire aligning substantially over the inner periphery of the first wire and the outer periphery of the second wire aligning substantially over the outer periphery of the first wire. The shorted turn includes a third wire contained in a plane having an inner and an outer periphery, and is a closed loop, the inner periphery of the third wire aligning substantially over the inner periphery of the second wire and the outer periphery of the third wire aligning substantially over the outer periphery of the second wire. The fuse includes a first insulator between the first wire and the second wire and a second insulator between the second wire and the third wire.
This structure and process is superior to conventional fuses because the fragile low K dielectric is not exposed to the high internal pressures and high local temperatures that are part of the laser deletion of a segment of metal line. Therefore, blowing the fuse does not damage the surrounding dielectric material. Further, the active link of the inductive fuse is isolated from the remainder of the fuse and device circuitry, i.e. there is no physical connection between that feature and other circuits in the device. Once opened, the inductive fuse and the remainder of the device wiring is insensitive to corrosion of the exposed, opened fuse element. Corrosion in the form of dendritic growth cannot occur as there is no DC bias across the opened fuse structure. Corrosion in the form of metal conversion (oxidation) is isolated to the active fuse link.